The present invention relates to a power hand tool, and more particularly to a bit holding system for a power hand tool which can be controlled to provide a high torsional force intermittently.
FIG. 1 shows a power hand drill according to the prior art. According to this structure of power hand drill, a transmission block is coupled to the motor shaft of a reversible motor drive, a plurality of clamping rods are obliquely mounted around the transmission block, each clamping rod having a threaded inner end threaded into a respective screw hole on the transmission block, and a control ring for adjusting the position of the clamping rods. The control ring has a serrated front edge. When turning the control ring to adjust the position of the clamping rods, a special T-shaped driving tool is used and inserted into a positioning hole on the casing of the power hand drill to force a bevel gear of the T-shaped driving tool into engagement with the serrated front edge of the control ring, and therefor the control ring is turned when rotating the T-shaped driving tool. Because the T-shaped driving tool must be used to adjust the position of the clamping rods when mounting or dismounting the bit, the T-shaped driving tool must be well kept with the power hand drill.